The objective of this proposal is to model the effect of tumor size and growth in promoting tumor progression. More specifically, the proposal aims to create a micro fabricated platform that will allow generation of uniform, size-controlled micro tumors in a high throughput manner and study size-dependent differences in tumor microenvironment, cell-cell interaction and transcription factors. Recently tumor microenvironment has been considered as an important player in tumorigenesis and progression leading to metastasis, a major cause of cancer-related mortality. Tumor microenvironment consists of tumor & stromal cells, extracellular matrix and non-cellular components that include hypoxia, interstitial pressure and metabolic stresses. Growing tumor exerts forces on the surrounding cells and epithelium creating radial and circumferential stress. Tumor growth also results in metabolic stress, hypoxia, necrosis, increased matrix stiffness and interstitial fluid pressure. Although tumor size has been considered as one of the important predictor of metastasis and patient survival, the mechanistic link between tumor size and the clinical outcome (metastasis, mortality) is not well understood. In breast cancer patients, this scenario is complicated by nodal and hormonal receptor status. This proposal aims 1) to develop three-dimensional (3D) micro fabricated platform to generate size-controlled micro tumors with controlled microenvironments using sub-type specific breast cancer cell lines and 2) to understand the effect of tumor size on microenvironment and delineate the mechanism by which tumor size promotes tumorigenesis and metastasis. Aim 1 will use micro fabrication technologies to pattern cells in non-adhesive hydrogel micro wells and generate size-controlled micro tumors of different sizes. Various sub-type specific cell lines (luminal, basal, triple negative etc.) will be used to investigate the size-dependent differences in their growth kinetics, hormonal status, cell-cell interaction and tumor microenvironment. In Aim 2, size-dependent transcriptional changes in micro tumors will be studied and this information will be used further to understand the mechanism behind tumor size-related tumor progression using RNA interference and small molecule inhibitors. The proposed approach is novel, easy and inexpensive and will allow fabricating 3D micro tumor models in a high throughput manner. Ability to control microenvironments in size-dependent manner will facilitate discovery of key signaling mechanisms involved in the regulation of morphological and transcriptional changes in tumor cells. This approach can potentially be used as a more relevant 3D drug screening platform to replace current 2D cell monolayers.